1. Field of the Invention
This invention relates generally to electromagnetic signal receiving devices, and more particularly to a receiving antenna with a plurality of feeds for receiving microwave signals emanating from more than one direction.
2. Description of the Prior Art
Direct Broadcast Satellite (DBS) is a point-tom-ultipoint system in which individual households equipped with a small receiving antenna and tuner device receive broadcasts directly from a geostationary satellite. The satellite receives digital audio and video transmissions from ground stations and relays them directly to individuals. The receiving antenna is comprised of a parabolic dish designed to collimate the satellite signals at the focal point, where an LNBF (Low Noise Block with integrated Feed) module is mounted to convert the incoming signals to a lower frequency band and transmit it to a tuner device. The LNBF module also acts as a filter and an amplifier to selectively boost the signal received by the dish collector. The LNBF module comprises a feed for receiving microwaves and circuitry for processing the received microwaves.
Because of the high sensitivity of these devices and relatively high satellite transmitting power, the parabolic dish collector currently being used can be as small as 0.4 meter in diameter. The dishes are mounted outside the home and are manually aligned with a diagnostic display showing received signal strength. Inside the home, a phase-lock loop tuner demodulates the signal from the LNBF module into video and audio signals suitable for a television or stereo tuner.
Normally, each satellite dish antenna is aligned to receive signals from a particular cluster (or group) of satellites in a certain direction. To a dish antenna on earth, the satellites belonging to the same cluster are located so close together that their signals are indistinguishable from signals emanating from a single satellite. Microwave signals aligned to the axis of the parabolic antenna dish are collected at the focal point, where the LNBF module is located. Shown in FIG. 1 is a typical LNBF module. Much effort has been put into the design of the feed structure to increase dish gain, polarization isolation, and selectivity. Much research has also been done to find ways to increase bandwidth during transmission using modulation techniques, or using different polarization methods to transmit different channels of signals. When receiving signals from different satellite clusters, more than one dish antenna may be used to point to the different angles. Another method is to use an electric motor to turn the antenna assembly to point to different satellites. However, employing these methods would make the antenna too expensive for general home use.
When two satellites (or two clusters of satellites), are separated by a small angle (the angle being larger than the separation angle between satellites within the same cluster), it is possible to use two LNBF modules placed side by side near the focal point to receive signals from the two satellites. The separation between the feeds of the two LNBF modules is proportional to the separation angles of the two satellites and the focal length. With an F/D (dish focal length over dish diameter) ratio fixed, as the dish size is decreased to less than half a meter, it becomes very difficult to place two conventional horn feeds within the required distance without causing excessive spill over loss. The spill over loss will show up as a signal to noise ratio decrease which will affect signal reception quality.
To effectively match the LNBF feed to the dish antenna, the feed radiation pattern should have a gain drop of about -10 to -12 dB for signals coming from outside the dish boundary. The radiation field pattern of a horn feed is correlated with the width of the horn opening. The wider the opening is, the narrower the radiation field pattern will become. Narrower feed radiation pattern can better filter out unwanted signals and decrease spill over loss. It can also lessen the demand for a high antenna dish directivity.
Typically, for a dish collector receiving signals ranging from 12.2-12.7 GHz, having a 45 centimeter diameter, and a focal length of 20 centimeters, the optimized opening of the horn feed should be around 3.6 cm. If the opening is too narrow, then too much noise will be picked up by the horn feed, increasing the spill over noise. When the two satellites are separated by 4.5 degrees, for example, the separation between the opening centers of the two horn feeds should be about 2.35 cm. To reduce the spill over loss, a much wider antenna dish would be required for receiving signals from two satellite systems than that of receiving signals from a single satellite system.